One of the most convenient methods of determining such a refractive-index profile is the so-called near-field technique which measures the luminous energy at the output end of the fiber whose input end is being illuminated from a light source of uniform radiance (known as a Lambertian source), with suppression of slanting leaky-mode rays so that practically all the incident light is internally reflected by the sheath or cladding of the fiber core and is thus transmitted to the opposite end. The power distribution I(r) along a radius of the fiber, as determined by a scanning of its output end, is then given by EQU I(r)=k[n.sup.2 (r)-n.sub.e.sup.2 ] (1)
where n(r) is the refractive index at a given distance from the fiber axis, n.sub.e is the index of the cladding surrounding the core, and k is a proportionality factor.
The suppression of leaky-mode rays is a relatively simple matter when the fiber has a stepped profile, i.e. when its refractive index varies discontinuously in the radial direction; in that case the undesired rays all have an angle of incidence exceeding a predetermined value and can thus be eliminated with the aid of a suitable diaphragm. In the case of a graded or continuously varying profile, on the other hand, these leaky rays are practically inseparable from guided rays commingled therewith. Reference in this connection may be made to an article by K. I. WHITE in Optical and Quantum Electronics 11 (published 1979 by Chapman & Hall Ltd. in Great Britain), pages 185-196, titled "Practical application of the refracted near-field technique for the measurement of optical fibre refractive index profiled".
Another problem arising with this near-field technique is the fact that, in conformity with equation (1), a refractive index n(r) which is lower than the cladding index n.sub.e at any radius of the fiber core cannot be measured. The formula also fails with fibers of very small cross-section transmitting only in a single mode or in a low number of modes.
The above-identified Paper by K. I. White discusses an alternative method of determining the refractive-index profile, referred to as a "refracted near-field techinque", which measures lost instead of transmitted power and thus also takes the leaky rays into consideration. That method, however, requires certain adjustments to compensate for lack of precise linearity which must be determined empirically and are therefore not readily reproducible.